1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a transflective liquid crystal display device.
2. Discussion of the Related Art
A liquid crystal display device has characteristics of light weight, thin thickness, and low power consumption. Thus, it has been highlighted as a next generational display device. Generally, an LCD device is a non-emissive display device displaying images by using a refractive index difference according to the optical anisotropy of the liquid crystal interposed between array and color filter substrates.
In the conventional LCD device, a displaying method using a backlight behind the array substrate as a light source is commonly used. However, the incident light from the backlight is attenuated during the transmission so that the actual transmittance is only about 7%. The backlight of the conventional LCD device requires high brightness, and thus power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device, and the battery cannot be used for a long period of time.
In order to overcome the problems described above, a reflective LCD has been developed. Since the reflective LCD device uses the ambient light instead of the backlight, it becomes light and easy to carry. In addition, power consumption of the reflective LCD device is reduced so that the reflective LCD device can be used for a portable display device such as an electric diary or a personal digital assistant (PDA).
However, brightness of the reflective LCD device may vary with the surroundings. For example, brightness of the indoor ambient light differs largely from that of the outdoors. Therefore, the reflective LCD device cannot be used where the ambient light is weak or does not exist. In order to overcome these problems, a transflective LCD device has been researched and developed. The transflective LCD device is switchable according to the user""s selection from a transmissive mode using transmission of light to a reflective mode using reflection of light.
To increase a light efficiency between the transmissive and reflective modes, retardations (xcex4) of liquid crystal layer of the transmissive and reflective modes should be equal. The retardation of the liquid crystal layer is defined by the following equation:
xcex4=xcex94nxc2x7d
wherein xcex4 is a retardation of the liquid crystal layer, xcex94n is a refractive index anisotropy of the liquid crystal layer, and d is a cell gap of the liquid crystal layer.
Therefore, the retardation of the liquid crystal layer in the transflective LCD device may be constant by forming a cell gap of the transmissive portion larger than that of the reflective portion.
FIG. 1 is a schematic cross-sectional view of a conventional transflective LCD device.
In FIG. 1, upper and lower substrates 10 and 30 are spaced apart from each other and a liquid crystal layer 20 is interposed therebetween. A backlight 38 is disposed at the outside of the lower substrate 30. On the inner surface of the upper substrate 10, a color filter layer 12 for passing only the light having a specific wavelength and a common electrode 14 functioning as one electrode for applying a voltage to the liquid crystal layer 20 are subsequently formed. On the inner surface of the lower substrate 30, a transparent pixel electrode 32 functioning as another electrode for applying a voltage to the liquid crystal layer 20, a passivation layer 34 having a transmissive hole 31 exposing a portion of the pixel electrode 32, and a reflective layer 36 are subsequently formed. The area corresponding to the reflective layer 36 is a reflective portion xe2x80x9crxe2x80x9d and the area corresponding to the portion of the pixel electrode 32 exposed by the transmissive hole 31 is a transmissive portion xe2x80x9ctxe2x80x9d.
A cell gap xe2x80x9cd1xe2x80x9d at the transmissive portion xe2x80x9ctxe2x80x9d is about twice of a cell gap xe2x80x9cd2xe2x80x9d at the reflective portion xe2x80x9crxe2x80x9d to reduce a light path difference. However, even though the light efficiency of the liquid crystal layer between reflective and transmissive modes becomes equal by making the cell gap different, the number of light passing through the color filter layer at different portions is different. Thus, the brightness becomes different at the front of the display device.
Transmittance of the color filter resin having a high absorption coefficient only for a specific wavelength satisfies the following equation when Fresnel reflection is not considered and the transmittance is inversely proportional to the absorption coefficient and the distance that light passes:
T=exp(xe2x88x92xcex1xc2x7d)
wherein T is transmittance, xcex1is an absorption coefficient of the color filter layer and d is a distance that light passes in the color filter layer.
At the reflective portion xe2x80x9crxe2x80x9d, light passes the color filter layer 12 twice. Since the transmittance and the color purity are determined by an absorption coefficient and a thickness of the color filter layer according to the above equation, the values of exp(xe2x88x92xcex1xc2x7d) at the transmissive and reflective portions should be controlled to be equal to avoid differences of the transmittance between the transmissive and reflective portions. Therefore, the transmittance becomes constant at the transmissive and reflective portions by forming the color filter layer of the reflective portion thicker than that of the transmissive portion with the same absorption coefficient. For example, the color filter layer at the reflective portion is formed to be twice as thick as that at the transmissive portion. Alternatively, the absorption coefficient of the color filter layer at the reflective portion is formed to be lower than that at the transmissive portion.
Generally, color purity increases and transmittance decreases when a color filter layer becomes thicker. Therefore, the transmittance and the color purity of the transmissive and reflective portions are maintained by increasing the transmittance and decreasing the color purity of the reflective portion, or by increasing the color purity and decreasing the transmittance of the transmissive portion.
A color filter layer is classified into a dye type and a pigment type depending on the material of the organic filter. Depending upon the method of fabricating the color filter layer, it may also be divided into a dyeing method, a printing method, a pigment dispersion method, and an electro-deposition method. The pigment dispersion method is most widely employed.
FIG. 2A is a cross-sectional view of a conventional transflective LCD device having a color filter layer. Pigment concentrations at the transmissive and reflective portions of the color filter layer are different from each other. Since the structure of FIG. 2A is similar to that of FIG. 1, the explanation about the same structure will be omitted for convenience.
In FIG. 2A, reflective and transmissive color filters 40a and 40b are disposed at the reflective and transmissive portions xe2x80x9crxe2x80x9d and xe2x80x9ctxe2x80x9d, respectively. The transmittance of the reflective and transmissive color filters 40a and 40b becomes different from each other when the absorption coefficients of the reflective and transmissive color filters 40a and 40b are adjusted. For example, the pigment concentrations of the reflective and transmissive color filters 40a and 40b may be formed differently from each other. Since the pigment concentration of the color filter layer is proportional to the absorption coefficient of the color filter layer, the transmittance of the reflective color filter layer 40a can be higher than that of the transmissive color filter layer 40b by making the pigment concentration of the reflective color filter layer 40a lower than that of the transmissive color filter layer 40b. 
FIG. 2B is a cross-sectional view of a conventional transflective LCD device having a color filter layer whose thicknesses at the transmissive and reflective portions are different from each other. Since the structure of FIG. 2B is similar to that of FIG. 1, the explanation about the same structure will be omitted for convenience. A color filter having two different thicknesses at the reflective and transmissive portions of the same color filter layer can be referred to as a dual thickness color filter (DCF) type.
In FIG. 2B, a transmissive color filter layer 42b is thicker than a reflective color filter layer 42a by forming a transparent buffer layer 44 on the inner surface of an upper substrate 1 at the reflective portion xe2x80x9crxe2x80x9d. Since the thicknesses of the reflective and transmissive color filter layers 42a and 42b are varied with the thickness of the buffer layer 44, a desired thickness ratio between the reflective and transmissive color filter layers 42a and 42b is obtained. Therefore, transmittance and color purity of the reflective and transmissive portions are adjusted by making the color purity of the transmissive color filter layer higher than that of the reflective color filter layer.
As shown in FIGS. 2A and 2B, a color filter layer of a transflective liquid crystal display device has different color purity at reflective and transmissive portions. The different color purity of the reflective and transmissive portions can be formed by using thickness difference of a color resin or by using concentration difference of a color resin. The color filter layer having the different color purity is referred to as a transflective color filter layer.
FIG. 3 is a schematic perspective view showing a transflective color filter layer of an upper substrate and a transflective portion of a lower substrate for a conventional transflective LCD device.
In FIG. 3, the upper and lower substrates are attached to each other without a misalignment. Generally, an attachment error margin is determined by the substrate design and should be within a range of a few micrometers. If the upper and lower substrates are attached with a misalignment greater than the error margin, a desirable driving characteristic may not be obtained due to a light leakage. Therefore, the attachment process should be performed to be within the error margin.
In FIG. 3, a transflective portion 46 of the lower substrate and a transflective color filter layer 48 of the upper substrate are spaced apart from each other. A lower transparent electrode 46b and a transmissive color filter layer 48b are disposed at the center of the transflective portion 46 and transflective color filter layer 48, respectively. A reflective layer 46a and a reflective color filter layer 48a surround the lower transparent electrode 46b and the transmissive color filter layer 48b, respectively. Therefore, in the conventional LCD device, the boundary of the transmissive and reflective color filter layers of the upper substrate should be aligned with the boundary of the transmissive and reflective portions of the lower substrate.
When there is no attachment error margin, the border of the transflective color filter layer 48 should be exactly aligned with the border of the transflective portion 46 of the lower""substrate. Thus, designed values of transmittance and color purity in the transmissive and reflective modes are different from the actual values when a misalignment occurs.
FIG. 4 is a schematic plane view showing ideal alignment and misalignment states for a conventional transflective LCD device.
In FIG. 4, a first borderline xe2x80x9cAxe2x80x9d shows the border of reflective and transmissive color filter layers 50a and 50b, and second and third borderlines xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d show the borders of reflective and transmissive portions of a lower substrate, which are moved from the first borderline xe2x80x9cAxe2x80x9d due to a misalignment. The misalignment of the third borderline xe2x80x9cCxe2x80x9d is greater than that of the second borderline xe2x80x9cBxe2x80x9d. In the conventional LCD device having the structure of FIG. 4, since a color filter layer having different transmittance and color purity is included in the opposite color filter layer due to a misalignment during the attachment process, the transmittance increases and the color purity decreases at the transmissive portion, and the transmittance decreases and the color purity increases at the reflective portion. Therefore, the display quality is deteriorated.
Accordingly, the present invention is directed to a transflective liquid crystal display device that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide a transflective liquid crystal display device having an improved display quality by maintaining transmittance and color purity to be a desired value even when a misalignment occurs.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a transflective liquid crystal display device includes first and second substrates facing into each other, a pixel electrode on an inner surface of the second substrate, the pixel electrode having a first borderline between the reflective and transmissive portions, a transflective color filter layer on an inner surface of the first substrate, the transflective color filter layer having a second borderline between the reflective and transmissive portions, the first and second borderlines being not aligned in a vertical direction and separated by a distance to be within an attachment error margin, a transmittance of the transflective color filter layer at the reflective portion being higher than that of the transflective color filter layer at the transmissive portion, a common electrode on the transflective color filter layer, and a liquid crystal layer between the pixel electrode and the common electrode.
In another aspect of the present invention, a transflective liquid crystal display device includes first and second substrates facing into each other, a pixel electrode on an inner surface of the second substrate, the pixel electrode having a first borderline between the reflective and transmissive portions, a transflective color filter layer on an inner surface of the first substrate, the transflective color filter layer having a plurality of light passing regions without filtering light between the reflective and transmissive portions, a common electrode on the transflective color filter layer, and a liquid crystal layer between the pixel electrode and the common electrode.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.